The present disclosure relates to a semiconductor optical device. For example, a semiconductor optical device represented by a semiconductor laser element or a light-emitting diode has a multilayer structure including a first compound semiconductor layer having a first conductivity type, an active layer, and a second compound semiconductor layer having a second conductivity type different from the first conductivity type. A second electrode is formed on the second compound semiconductor layer. The first compound semiconductor layer is electrically connected to a first electrode.
FIG. 5 (Source: Ishikawa et al., J. Appl. Phys. 81. 1315 (1997)) illustrates a relationship between a metal constituting a second electrode (p-side electrode) formed on a second compound semiconductor layer formed of p-type GaN and a resistance value. Incidentally, a black circle indicates a value before an annealing treatment, and a white circle indicates a value after an annealing treatment at 500° C. A metal having a larger work function value used as the second electrode with respect to the second compound semiconductor layer formed of p-type GaN has a lower Schottky barrier, and therefore tends to obtain a better contact characteristic. FIG. 5 indicates that a resistance value is reduced according to increase in a work function value of a metal. Therefore, in general, a platinum group metal such as Pd or Pt, Ni, Au, or the like having a work function value distributed in a range of 5 eV to 5.65 eV is used as an ohmic electrode with respect to the second compound semiconductor layer formed of p-type GaN.
For example, JP 10-242587 A discloses a nitride semiconductor element for reducing a driving voltage. This nitride semiconductor element includes a p-type electrode formed on a p-type nitride semiconductor layer through a p-type contact layer. In addition, the p-type contact layer is formed of a p-type nitride semiconductor having a smaller band gap energy than the p-type nitride semiconductor layer, and has a thickness of 50 nm or less.    Patent Document 1: JP 10-242587 A